The present invention relates to a three-dimensional structure which includes an anti-reflection structure in a three-dimensional photonic crystal having a three-dimensional periodic structure, and a light-emitting device using the same.
Yablonovitch has suggested in “Physical Review Letters, Vol. 58, pp. 2059, 1987” that a periodic structure called a photonic crystal having a period equal to or shorter than an incident wavelength enables control of transmission and reflection characteristics of light as an electromagnetic wave.
Use of the photonic crystal having a so-called photonic band gap enables realization of an optical element having a new function. For example, a point defect or a line defect provided in the photonic crystal serves as a resonator or a waveguide.
When the line defect serving as the waveguide is provided in the photonic crystal, light propagates therein in a state of having a unique electromagnetic energy distribution according to a structure of the waveguide. Outside the photonic crystal, light propagates in a state of having a unique electromagnetic energy distribution according to a structure of the outside.
Hereinafter, the state of the light propagating with a unique electromagnetic energy distribution is referred to as a guide mode of the light. Further, a unique electromagnetic energy distribution in a certain guide mode is referred to as a guide mode pattern. A waveguide in the photonic crystal is referred to as a waveguide 1, and a guide mode of light propagating in the waveguide 1 is referred to as a guide mode 1.
The light of the guide mode 1 propagating in the waveguide 1 in the photonic crystal is coupled with light propagating in a guide mode (guide mode 2) different from the guide mode 1, in other words, the light of the guide mode 1 is made usable by converting the guide mode. Hereinafter, a rate at which at least part of energy of the light propagating in the guide mode 1 is converted into energy of the light propagating in the guide mode 2 when the light propagating in the guide mode 1 is coupled with the light propagating in the guide mode 2 is referred to as coupling efficiency.
When the waveguide 1 in the photonic crystal is connected with a structure in which light propagates in the guide mode 2, part of the light propagating in the waveguide 1 in the guide mode 1 is coupled with the light propagating in the guide mode 2. Further, part of the light propagating in the guide mode 1 becomes a reflected wave to propagate in the waveguide 1.
In order to convert the light propagating in the guide mode 1 into the light propagating in the guide mode 2 to use the converted light efficiently, improvement of the coupling efficiency between the light propagating in the guide mode 1 and the light propagating in the guide mode 2 and reduction of the reflected wave propagating in the waveguide 1 are required.
To meet such requirements, Japanese Patent Laid-Open No. 2003-315572 discloses an example in which a waveguide 2 as a tapered defect is formed between a waveguide 1 and a free space in a photonic crystal by gradually increasing a width of a line defect. A guide mode of light propagating in the free space is referred to as a guide mode 2, while a guide mode of light propagating in the waveguide 2 formed by the tapered defect is referred to as a guide mode 3.
In Japanese Patent Laid-Open No. 2003-315572, connecting the waveguide 2 with the waveguide 1 and the free space causes the light of the guide mode 1 propagating in the waveguide 1 to be converted into light of the guide mode 3 having a pattern shape similar to that of the guide mode 2, thereby causing the light of the guide mode 3 to be coupled with the light of the guide mode 2. Thus, the coupling efficiency between the light propagating in the waveguide 1 in the guide mode 1 and the light propagating in the free space in the guide mode 2 can be improved, and a reflected wave propagating in the waveguide 1 can be reduced.
Further, Japanese Patent Laid-Open No. 2003-270458 discloses a structure in which, between two waveguides having guide modes different from each other, a third waveguide is provided which has a guide mode different from the guide modes of the two waveguides. Specifically, the structure includes between a waveguide 1 provided in a photonic crystal and a waveguide 2 provided outside the photonic crystal the third waveguide (waveguide 3) whose structure is different from that of the waveguide 1. The guide mode of light propagating in the waveguide 2 is a guide mode 2, while the guide mode of light propagating in the waveguide 3 is a guide mode 3. Part of light propagating in the waveguide 1 becomes a reflected wave at a connection portion of the waveguides 1 and 3 due to the difference of the guide modes 1 and 3. Part of light propagating in the waveguide 3 becomes a reflected wave at a connection portion of the waveguides 2 and 3 due to the difference of the guide modes 2 and 3.
Japanese Patent Laid-Open No. 2003-270458 discloses that interference of these two reflected waves reduces the reflected wave propagating in the waveguide 1, thereby improving the coupling efficiency between the light propagating in the waveguide 1 in the guide mode 1 and the light propagating in the waveguide 2 in the guide mode 2.
In the structure disclosed in Japanese Patent Laid-Open No. 2003-315572, the guide mode 1 of the light propagating in the waveguide 1 in the photonic crystal and the guide mode 3 of the light propagating in the waveguide 2 formed by the tapered defect are mutually different guide modes. Therefore, connection of the waveguides 1 and 2 causes part of the light propagating in the waveguide 1 in the guide mode 1 to become a reflected wave. This reflected wave is a loss since it is not coupled with the light propagating in the guide mode 3. In other words, in the connection portion of the waveguides 1 and 2, part of the light propagating in the guide mode 1 cannot be suppressed from becoming the reflected wave.